Embodiments relate to a semiconductor device, and more particularly, to a CMOS image sensor and a fabricating method thereof. As generally known, an image sensor may be a semiconductor device that converts an optical image to an electric signal. Image sensors may be classified into charge coupled devices (CCD) and complementary metal-oxide semiconductor (CMOS) image sensors. CMOS image sensors use a switching method that employs a MOS transistor for each pixel. CMOS sensors also simultaneously integrate a peripheral control circuit, and a signal processing circuit. The sensor outputs the signal through the MOS transistor. The CMOS image sensor therefore includes a photo diode and a plurality of MOS transistors.
FIG. 1 is a sectional view of a related CMOS image sensor. Referring to FIG. 1, over a semiconductor substrate, for example over a Si substrate 10, a plurality of photo diodes 22, 24 and 26 receive, respectively, red, green and blue light. Photo diodes 22, 24 and 26 may be formed by repeating an ion implantation process and a Si epitaxial growth process.
More particularly, an ion implantation layer for the red photodiode 22 may be formed first over the substrate 10. A first Si epitaxial layer 12 may be formed over the ion implantation layer. The green photodiode 24 may be formed in the first Si epitaxial layer 12 by ion implantation. In addition, a plug 42 for contacting the red photodiode 22 may be selectively formed over the first Si epitaxial layer 12. Next, a second Si epitaxial layer 14 may be formed using the Si epitaxial growth method. Here, the blue photodiode 26 may be formed over the second Si epitaxial layer 14. Additionally, another plug 46 for contact with the green photodiode 24 and yet another plug 44 for contact with the red photodiode 22 may be formed in the Si epitaxial layer 14. Over the second Si epitaxial layer 14, a plurality of the MOS transistors may be formed to transfer photocharges collected by the photodiodes 22, 24 and 26. The plurality of MOS transistors may include a gate 30, a gate dielectric layer 32, and a spacer 34.
In the above-described CMOS image sensor, the red photodiode 22 and the green photodiode 24 occupy a greater area than the blue photodiode 26. However, since the transistors for signal transmission are disposed on the uppermost part of the CMOS image sensor, only the area occupied by the blue photodiode 26 is fully effective as a light receiving area. As a result, the effective light receiving area is smaller than the area occupied by the photodiodes 22, 24 and 26.
Another issue is the plugs 42, 44 and 46 for processing signals of the red photodiode 22 and the green photodiode 24, which may be formed by ion implantation. When light is received from the outside, the plugs 42, 44 and 46 may disrupt and create noise in the light signal. Furthermore, the red, green and blue photodiodes 22, 24 and 26 should be separately formed to prevent interference with one another. To address this issue, a dedicated ion implantation layer for separate photodiodes may be required in the respective epitaxial layers.